Final answer:
Avascular necrosis is the most likely cause of hip pain in a patient with a long history of severe Crohn's disease on steroids.
Step-by-step explanation:
The most likely cause of hip pain in a patient with a long history of severe Crohn's disease on steroids would be avascular necrosis. Avascular necrosis occurs when the blood supply to the bones in the hip joint is disrupted, leading to bone and cartilage death. This can be a side effect of long-term steroid use. Other potential causes of hip pain in this patient could include inflammatory arthritis and osteoporosis, but avascular necrosis is the most likely culprit.